The present invention relates generally to noncontact gauging systems. More particularly, the invention relates to an apparatus system and method for calibrating noncontact gauging systems.
Demand for higher quality has pressed manufacturers of mass produced articles, such as automotive vehicles, to employ automated manufacturing techniques that were unheard of when assembly line manufacturing was first conceived. Today, robotic equipment is used to assemble, weld, finish, gauge and test manufactured articles with a much higher degree of quality and precision than has been heretofore possible. Computer-aided manufacturing techniques allow designers to graphically conceptualize and design a new product on a computer workstation and the automated manufacturing process ensures that the design is faithfully carried out precisely according to specification. Machine vision is a key part of today's manufacturing environment. Machine vision systems are used with robotics and computer-aided design systems to ensure high quality is achieved at the lowest practical cost.
Achieving high quality manufactured parts requires highly accurate, tightly calibrated machine vision sensors. Not only must a sensor have a suitable resolution to discern a manufactured feature of interest, the sensor must be accurately calibrated to a known frame of reference so that the feature of interest may be related to other features on the workpiece. Without accurate calibration, even the most sensitive, high resolution sensor will fail to produce high quality results.
In a typical manufacturing environment, there may be a plurality of different noncontact sensors, such as optical sensors, positioned at various predetermined locations within the workpiece manufacturing, gauging or testing station. The workpiece is placed at a predetermined, fixed location within the station, allowing various predetermined features of the workpiece to be examined by the sensors. Preferably, all of the sensors properly positioned and should be carefully calibrated with respect to some common fixed frame of reference, such as a common reference frame on the workpiece or at the workstation.
Keeping sensors properly positioned and calibrated is more easily said than done. In a typical manufacturing environment sensors and their associated mounting structures may get bumped or jarred, throwing the sensor out of alignment. Also, from time to time, a sensor needs to be replaced, almost certainly requiring reorienting and recalibrating. Quite simply, sensor positioning, alignment and calibration is a fact of life in the typical manufacturing plant.
The problem with sensor positioning, alignment and calibration is the time required. Invariably, the entire manufacturing assembly line for a given part must be shut down and the workstation cleared, so that the sensor may be positioned, aligned and recalibrated. In some instances this entails placing a highly accurate (and very expensive) full-scale model of the workpiece in the workstation. This independently measured part is sometimes called a master part. The master part is placed in careful registration with the external coordinate system of the workstation and then each sensor is trained on its assigned feature (such as a hole or edge). Once positioned, the sensors are locked into place and calibrated and the master part is removed. Only then can the assembly line be brought back online.
As an alternative to using a master part, it is possible to calibrate the gauging sensor by attaching a target to the sensor and illuminating the target using a plane of structured light produced by the sensor. A pair of optical sighting devices, theodolites, are placed at different vantage points within the workspace. The theodolites triangulate on the illuminated target to provide an independent reading of the position of the target. The theodolites are placed at carefully prescribed locations relative to the external reference frame. With the gauging sensor projecting structured light onto the target, the theodolites are manually aimed at the lighted targets and readings are taken. The respective readings of the theodolites and the gauging sensor are coordinated and translated to calibrate the sensor relative to the external reference frame. It is a trial and error process. If the sensor needs to be reoriented (as is often the case), the theodolites must be manually retrained on the target after each sensor position adjustment. For more information on this calibration technique, see U.S. Pat. No. 4,841,460 to Dewar et al.
Whereas both of the aforementioned calibration techniques do work, there is considerable interest in a calibration technique that is quicker and easier to accomplish and that eliminates the need to rely on expensive master parts or difficult to use theodolite equipment. To this end, the present invention provides a calibration system that can be used in a matter of minutes, instead of hours, and without the need for precisely manufactured master parts or theodolite equipment. One of the major advantages of the invention is that it allows the calibration of a sensors to be checked or realigned between line shifts, without requiring the line to be shut down for an extended period.
The system employs a portable reference target that has a retroreflector mounted at a known location with respect to the center of the tetrahedron. The retroreflector is designed to reflect light from a companion laser tracker that is servo controlled to track the position of the retroreflector with its laser beam. The laser tracker is thereby able to acquire the position of the retroreflector (and thereby acquire the position of the attached portable reference target).
The presently preferred portable reference target is a tetrahedron framework that provides at least three noncolinear and noncoplanar geometric structures (e.g., straight edges) that are illuminated by structured light emanating from the feature sensor. These noncolinear geometric features provide the feature sensor with unambiguous spatial data for measuring the spatial position and attitude of the target.
The system uses coordinate transformations for coordinating the feature sensor coordinate frame to the external reference frame. The system includes a coordinate transformation system for coordinating the reading from the laser tracker and from the feature sensor. The laser tracker would be calibrated to the external reference frame using reference indicia and commercially available software. Using the tetrahedron with the structured light measurement, the transformation from sensor to tetrahedron space allows the identification of the center of the retroreflector in the sensor coordinates. As the laser tracker is only a device capable measuring X, Y and Z, but not orientation, multiple samples are needed to define the sensor orientation. Establishing three or more non-collinear points in a similar process allows the identification of the feature sensor coordinate frame with respect to reference coordinate frame of the laser tracker in all six degrees of freedom (X, Y, Z, roll, pitch and yaw).
Using the calibration system of the invention, it is easy to calibrate a feature sensor. The retroreflector is first illuminated by the laser tracker at several locations and used by the coordinate translation system to calibrate the laser tracker to the external reference frame. Next the target is placed within the field of view of the sensor under calibration. The portable reference target is calibrated with respect to the reference frame of the laser tracker. The sensor is then calibrated by projecting structured light from the feature sensor onto the portable reference target. The structured light intersects the target, producing reflected light patterns at the edges of the target that are then read by the feature sensor. The coordinate translation system then performs the appropriate coordinate translation to map the reading of the feature sensor back to the external reference frame.
The entire calibration sequence can be performed quite quickly. The laser tracker and portable reference targets are both lightweight and easily positioned. Moreover, the entire calibration sequence may be performed rapidly. In most instances, all the calibration technician must do is position the reference target at several locations while the laser tracker acquires and stores its position, and by then placing the portable reference target in front of the feature sensor and then allow the system to do the rest.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.